The ImpressCMS Project was formed in late 2007 as a result of a division in the XOOPS community. The first beta release of ImpressCMS was introduced in January 2008 and ImpressCMS 1.0 Final was published in March 2008, establishing its new identity apart from XOOPS and incorporating bug fixes, security enhancements and new features. This was followed by the release of ImpressCMS 1.1 in October 2008. 
Version 1.2 was released almost one year later . Major changes were done to the core of the system, hence the longer development time. This version introduced the ImpressCMS Persistable Framework (IPF) in the core, the ‘Content’ core module was transformed into a stand-alone module, and the installation system had a substantial refresh. On the visual side a new theme was introduced, featuring an AJAX redirect for system messages.
Marking the end of a long development period, version 1.3 was released on 20 September 2011. The ImpressCMS 1.3 series is a total refactor of the core system aimed at modernising the code structure, making the system smaller, faster and use less memory. This is also be the last branch of ImpressCMS to provide compatibility with previous Xoops-based modules. The latest release of ImpressCMS is currently ImpressCMS version 1.3.11
The PHP ecosystem has gone through some important changes by now, with frameworks like Symfony, Yii and Zend defining a whole structure of classes that are written in a reusable way. Next to that, the Composer tool introduced the PHP world to the use of components from different vendors or providers. ImpressCMS v2 development started in 2013 to start to integrate these new ways of working with PHP into the core. The latest release is ImpressCMS 2.0 alpha 7
For a bit more about ImpressCMS' history check out the ImpressCMS Wikipedia page.